In the past, the surface treated steel sheet used as a can material was mainly tin-plated steel sheet such as tin-plated and lightly-coated steel sheet, or nickel plated steel sheet, and electrolytic chromium coated steel (ECCS).
Usually, the plated surfaces of these steel sheets are chemically treated to thereby secure adhesion to a lacquer or resin film.
At the present, almost all chemical treatment of the commercialized surface treated steel sheet for cans is dipping or cathodic electrolysis using an aqueous solution mainly comprised of dichromate or chromic acid or cathodic electrolysis.
As exceptional treatment, Japanese Patent Publication (A) No. 52-68832 and Japanese Patent Publication (A) No. 52-75626 disclose “cathodic-anodic electrolysis in a tin phosphate aqueous solution”, but the applications are limited to cans for powdered milk used as is with the inside surface uncoated.
The main reason why cathodic-anodic electrolysis is not used for beverage cans and food cans other than cans for powdered milk is that adhesion with an organic film like a lacquer or resin film is insufficient.
On the other hand, the chromium (III) oxide film obtained by dipping using an aqueous solution mainly comprised of dichromate or chromic acid or cathodic electrolysis has a large effect of improvement of the adhesion with an organic film. Various chemical treatments for replacing this have been studied, but none have been put into practical use.
For example, Japanese Patent Publication (A) No. 52-92837 discloses a method of anodic treatment in a phytic acid or phytate solution.
In recent years, numerous technologies for providing a tin-plating layer with a film using a silane coupling agent have been disclosed.
For example, Japanese Patent Publication (A) No. 2002-285354 discloses steel sheet or cans comprised of tin-plated steel sheet with Sn layers or Fe—Sn alloy layers on which layers of a silane coupling agent have been coated, while Japanese Patent Publication (A) No. 2001-316851 discloses tin-plated steel sheet comprised of a tin-plating layer on which an inner layer of a chemical converted film containing P and Sn and an outer layer of a silane coupling layer are provided.
Further, art similar to the art disclosed in Japanese Patent Publication (A) No. 2001-316851 is disclosed in Japanese Patent Publication (A) No. 2002-275643, Japanese Patent Publication (A) No. 2002-206191, Japanese Patent Publication (A) No. 2002-275657, Japanese Patent Publication (A) No. 2002-339081, Japanese Patent Publication (A) No. 2003-3281, Japanese Patent Publication (A) No. 2003-175564, Japanese Patent Publication (A) No. 2003-183853, Japanese Patent Publication (A) No. 2003-239084, Japanese Patent Publication (A) No. 2003-253466, an Japanese Patent Publication (A) No. 2004-68063.